1. Field of the Invention
This invention relates to a method for the purification of lactide which is a cyclic ester formed from two molecules of lactic acid.
2. Description of the Prior Art
Lactide which is a cyclic ester formed from two molecules of lactic acid has been known to have three forms; L-lactide molecule formed from two molecules of L-lactic acid, D-lactide molecule formed from two molecules of D-lactic acid, and meso-lactide molecule formed from L-lactic acid and D-lactic acid.
Lactide is an important compound as an intermediate for polylactic acid which has been attracting attention as a biodegradable plastic substance in recent years. It has been known that polylactic acid with a high molecular weight can be obtained by the ring-opening polymerization of lactide.
The nature of the polylactic acid is varied by the optical purity of the lactic acid monomer forming the polylactic acid, namely the optical purity of the polylactic acid, as well as by the molecular weight of the polylactic acid. It has been known that pure L- or D-polylactic acid excels L- or D-polylactic acid having a like molecular weight and a lower optical purity in terms of strength or formability. Perfect racemic lactic acid, namely, DL-polylactic acid formed from DL-lactic acid, excels optically active polylactic acid in terms of flexibility and, therefore, is expected to find utility in special applications.
Now, the methods heretofore introduced to the art for the synthesis of lactide will be described below.
Lactide is generally synthesized by the so-called reaction distillation method which uses polylactic acid with a relatively low molecular weight (lactic acid oligomer which will be referred to hereinafter as "prepolymer" in association with the reaction for synthesis of lactide) as an intermediate and which comprises the steps of cyclizing this intermediate at a temperature of not lower than 180.degree. C. in the presence of a catalyst thereby forming lactide which is a cyclic ester formed from two molecules of lactic acid and extracting this lactide in the form of vapor out of the reaction system. To be more specific, the lactide is synthesized by the following procedure using lactic acid as a raw material.
(1) Lactic acid is heated under a reduced pressure (generally below 20 mmHg) until dehydration and condensation thereof and converted into a prepolymer. The temperature of this heating is kept below 180.degree. C. for the purpose of preventing lactic acid from racemization.
(2) The prepolymer and SnO added thereto as a catalyst for the synthesis of lactide are together heated under a reduced pressure and left reacting at a temperature in the approximate range of 180.degree. to 230.degree. C. while removing the formed lactide vapor by distillation.
(3) The lactide vapor is cooled and collected. It is recovered in a liquid state at a temperature exceeding 60.degree. to 90.degree. C. lest it should condense.
In the dehydration and condensation of lactic acid at the step (1) mentioned above, the reaction part may preparatorily contain the catalyst for the synthesis of lactide at the step (2). The idea of adapting the steps (1) to (3) of the procedure mentioned above to proceed as a continuous operation carried out and consequently exalting the efficiency of synthesis constitutes a naturally conceived method.
The lactide vapor expelled by distillation from the reaction part for the synthesis of lactide at any of the steps mentioned above contains lactic acid monomer, lactic acid dimer (lactoyllactic acid), and water as impurities. Hereinafter, the lactide which contains these impurities will be referred to as "crude lactide".
Of these impurities, the lactic acid monomer and the lactic acid dimer are acid components. During the synthesis of the polylactic acid by the ring-opening polymerization of lactide, therefore, they inconvenience the severance of the polylactic acid chain and the consequent production of polylactic acid with a high molecular weight. The presence of water promotes the hydrolysis of polylactic acid or lactide and entails the formation of an acid component and, as a result, similarly inconveniences the production of polylactic acid having a high molecular weight.
Generally, these impurities are removed from the crude lactide by such a method as crystallization, extraction, or distillation to permit the production of purified lactide.
When the separation of lactide from these impurities is effected by ordinary crystallization, however, the yield by the crystallization is too low to be commercially acceptable satisfactorily for the production of lactide of high purity.
In the separation by distillation, lactic acid and lactide undergo thermal polymerization, hydrolysis, etc. during the course of distillation. Since their thorough separation is difficult and the yield of lactide is low, this method does not deserve to be rated as satisfactory.
The method disclosed in WO 92/00974 which comprises the steps of dissolving crude lactide in such a solvent as acetone which is compatible with water, adding cold water to the resultant solution thereby inducing crystallization of lactide, and separating the lactide crystals from the other components of the reaction solution is claimed to produce lactide of high purity. This method suffers from heavy loss due to the crystallization because the solubility of lactide in acetone is high. It has been already known to remove the acid components from the crude lactide by dissolving the crude lactide in a solvent incompatible with water and exposing the resultant solution to contact with water thereby inducing extraction of the acid components from the solution. This method, however, permits no sufficient separation of L-lactide or D-lactide from meso-lactide.
When L-form optically active lactic acid is used as the raw material for the crude lactide, the lactic acid inevitably undergoes racemization and gives rise to meso-lactide and a minute amount of D-lactide in addition to L-lactide because an amply high reaction temperature and an amply long retention time must be used generally for allowing the reaction to proceed fully.
When DL-lactic acid is used as the raw material for the crude lactide, it only naturally follows that the reaction produces meso-lactide in addition to DL-lactide which is an equivalent mixture of D-lactide and L-lactide.
For the purpose of synthesizing polylactic acid with a high molecular weight and having a high optical purity, it is necessary that the lactic acids which are components of the lactide being used as the raw material possess a high optical purity, namely the lactide itself possess a high optical purity.
Further, in the synthesis of DL-polylactic acid, the lactide as the raw material therefor is an equivalent mixture of L-lactide and D-lactide. It may contain meso-lactide without posing any problem from the compositional point of view. When lactide in a solid state is handled as an intermediate raw material, however, the solid lactide is so soluble as to impair the convenience of the handling thereof because meso-lactide has high hygroscopicity and a low melting point in the approximate range of 40.degree. to 42.degree. C.
In view of the various factors remarked above, it is desired in the manufacture of lactide to obtain lactide containing no meso-lactide. With respect to a measure to remove meso-lactide from lactide, no appreciable study has been heretofore made.